This invention relates to a pneumatic radial tire having an improved durability of a belt, and more particularly to a pneumatic radial tire for heavy duty use.
In a pneumatic tire, a plurality of belt plies are provided on the outer side of at least one carcass ply reinforced with carcass cords arranged at angles in the range of 90.degree..+-.20.degree. with respect to the circumferential direction of the tire the shape of the tire is stabilized because the tensile force of the belt layer when the tire is inflated. A conventional pneumatic tire for heavy duty use is designed so that, when the tire is fitted on a proper rim and inflated, the tensile force T is distributed uniformly across the entire widthwise direction as shown in FIG. 6. The tread surface of a conventional tire is shown by a broken line 11 in FIG. 3 in a meridional cross section of this pneumatic radial tire for heavy duty use at 5% of a normal inner pressure. The tire grows at the tread surface as a whole in the radially outward direction of the tire substantially uniformly as shown by a solid line 12, after the tire has been inflated to a normal inner pressure.
In a conventional radial tire, the tensile force T is thus imparted uniformly to the entire belt layer, so that an extremely high inter-belt-layer shearing force is exerted repeatedly on the widthwise edge portions of the belt layer during the travelling of the vehicle. In consequence, heat is generated in the edge portions of the belt layer tending to separate the cords from coating rubber. This decreases the durability of the belt.
In order to prevent the generation of heat in the edge portions of the belt, a compound (rubber composition) of a low heat build-up has been used as tread rubber, or the gauge thickness of the shoulder portions has been reduced. However, when a compound of a low heat build-up is used, the wet gripping capability (road surface grasping capability of the tire during the travelling of the vehicle on a wet road) of the tire decreases. When the gauge thickness of the shoulder portions is reduced thoughtlessly, the uneven-abrasion resistance of the tire lowers, and the shoulder portions wear earlier.